1. Field of the Invention
The present invention relates to an image forming apparatus employing electrophotographic system such as a copying machine or a printer.
2. Description of the Related Art
In an image forming apparatus employing electrophotographic system as a fixing unit for fixing an unfixed toner image to a recording material, a heat roller type and a film fixing type is widely used.
In the above-described fixing unit, when small-sized recording materials whose width is smaller than that of a recording material of a maximum width which the apparatus can convey, are to be successively conveyed, there is a problem called non-passing part temperature rise, in which the temperature of the non-passing area of the fixing member excessively increases.
As a countermeasure against the problem, Japanese Patent Application Laid-Open No. 4-51179 discusses a fixing unit in which cooling air is selectively supplied to the non-passing areas at both end portions of the fixing unit from an air supplying fan arranged in the fixing unit.
Further, Japanese Patent Application Laid-Open No. 2003-076209 discusses a fixing unit in which when supplying cooling air to the non-passing areas from the air supplying fan, the length in the width direction of the air supplying fan outlet is adjusted according to the width of the recording material in a direction orthogonal to the recording material conveyance direction, whereby it is possible to prevent an increase in temperature in the non-passing parts even in the case of recording materials differing in the above described width.
However, in the case of the fixing unit in which a cooling system to supply air to the non-passing areas of the fixing unit by providing the above-described air supplying fan is mounted, the following problems are involved.
First, the sheet is conveyed while being deviated with respect to a conveyance reference position in a direction orthogonal to the recording material conveyance direction. When above described deviated sheet-passing occurs, an image defect is generated at the edge portions of the recording material in the direction orthogonal to the recording material conveyance direction. FIGS. 13A and 13B are diagrams illustrating the positional relationship between the recording material and the fixing unit in which the above-described cooling system is mounted. FIG. 13A illustrates the state in which the recording material is conveyed at conveyance reference position. The diagram illustrates a recording material 2001, a film 2004, and air supplying fans 2002 and 2003. The recording material 2001 is conveyed such that a broken line M extending through the central portion of the film 2004 becomes a conveyance reference position. In the film 2004, non-passing part temperature rise occurs in areas G and H where the recording material 2001 does not pass. By supplying air to the areas G and H by the air supplying fans 2002 and 2003, respectively, it is possible to suppress the non-passing part temperature rise.
On the other hand, FIG. 13B illustrates a deviated sheet-passing state, in which the recording material is conveyed while being deviated from the conveyance reference position. Suppose the recording material 2001 is conveyed while being deviated in the direction close to the air supplying fan 2002 and orthogonal to the recording material conveyance direction. Non-passing part temperature rise occurs in an area I and an area K+H. The problem with this situation is that the recording material 2001 passes an area J which is cooled by the air supplying fan 2002, so that the area J does not reach a temperature enough to fix the toner to the recording material, resulting, in some cases, in occurrence of defective fixing. Further, although the area K is an area where non-passing part temperature rise occurs, no cooling air is supplied thereto, so that the temperature rise ratio is higher than in the other areas. Thus, it can be seen that when air is supplied by the air supplying fan at the time of generation of deviated sheet-passing, the difference in temperature between the area J and the area K increases as compared with the case where no air is supplied. In particular, in the case of the film fixing type, when an unsymmetrical temperature distribution is generated in the film, the thermal expansion amount of the pressure roller is also unsymmetrical. As a result, the difference in conveyance force between both end portions of the film in the direction orthogonal to the recording material conveyance direction is also large, resulting in occurrence of deviation of the film. As a result, the end portions of the film reach fracture in some cases.
Second, in the case where imbalance in the amount of air supplied to both end portions of the film is generated due to abnormal operation of the cooling system, the end portions of the film can reach fracture for the same reason as described above. Such imbalance in the amount of air supplied to both end portions occurs, for example, due to deterioration in the characteristics of the air supplying fans individually provided at both end portions of the film.